The present invention relates to an electrostatic copying apparatus comprising improved means for separating a copy sheet from a photoconductive drum after toner image transfer.
A copy sheet may be separated from a drum by means of a conductive member as disclosed, for example, in U.S. Pat. No. 3,508,824. The conductive member is grounded and the copy sheet engages therewith after toner image transfer. The electrostatic charge of the copy sheet induces an opposite electrostatic charge on the conductive member which causes the copy sheet to be attracted to the conductive member and separated from the drum thereby.
A problem which is inherent in this arrangement is that the conductive member discharges the copy sheet and causes the toner image to be smeared or transferred back to the drum. For this reason, it has been practiced to connect a constant voltage element such as a Zener diode and a resistor between the conductive member and ground. In this case, although the leading edge of the copy sheet is separated from the drum due to the initially zero voltage on the conductive member, a voltage is developed across the constant voltage element due to induced charge from the copy sheet of the opposite polarity which causes the toner image to adhere to the copy sheet after separation from the drum.
This latter expedient does not completely overcome the problem, however, due to the delay in developing the voltage across the constant voltage element. This causes a leading edge portion of the toner image to be transferred back to the drum and lost. The problem is complicated further due to the fact that the rate of increase of voltage across the constant voltage element up to the constant voltage level depends on the pattern of the toner image, different sizes and thicknesses of copy sheets and other factors which vary to a large extent.